The present invention is directed generally to novel systems and methods for performing thin film deposition or chemical treatment of substrates, and to optical devices manufactured using such systems and methods.
U.S. Pat. No. 6,402,904 (the ""904 patent) and U.S. patent application Ser. No. 09/810,688 filed Mar. 16, 2001, entitled xe2x80x9cSystem and Method for Performing Sputter Deposition Using Ion Sources, Targets and a Substrate Arranged About the Faces of a Cubexe2x80x9d (also incorporated herein by reference) (the ""688 application) disclose systems and methods for ion sputter deposition of thin films on a substrate which involve directing ions from at least one plasma ion source generally towards at least one sputter target, each target having its own associated ion source. In operation, negative voltages applied to the target(s) attract ions from the plasma and accelerate the ions toward the target to sufficient kinetic energies (50 to 5000 eV) to cause sputtering of the target. Electron sources may also be provided. For reasons of avoiding charge build-up on insulating targets, the negative voltages may be pulsed or even alternately pulsed with positive pulses which serve to attract electrons to the target to neutralize it.
The ""904 patent discloses yet another ion source directed generally at the substrate, and its main purpose is to bombard the growing film on the substrate with ion species chemically reactive with the sputter-deposited atoms from the target(s) to form compound thin films. For example O2+, N2+, H2+ and other ions collide with the growing film surface, dissociate and chemically react with the depositing atom flux sputtered from targets composed of pure Si, Al, Ti, Ta and others. In their respective combinations, compound thin films such as SiO2, Si3N4, SixH1xe2x88x92x, Al2O3, AlN, TiO2, TiN, Ta2O5, TaN and many others may be formed on the substrate. In the prior art, such an added ion source directed at the substrate enables so-called ion-assisted deposition (IAD).
IAD has many benefits. IAD supplies highly reactive O, N, H and other atomic species via surface-collisional dissociation of molecular ions, while the parent molecular species would not necessarily react (e.g., N2). This makes it easy to attain complete stoichiometry of various oxides, nitrides, hydrides and other compounds. IAD may also affect film growth by momentum-transfer physics, even with inert gas ions. IAD adds several controllable parameters to the process of thin film deposition. These are IAD ion species (and mixtures thereof), ion mass, ion kinetic energy, ion current density and ion incidence angle. Singly and in combination, variations in these parameters generally have a strong effect on film density, degree of crystalline versus amorphous character of the film, intrinsic film stress, refractive index, crystalline texture (grain orientation), crystalline grain size, grain boundary morphology, surface flatness/roughness and others. In addition, IAD may have the effect of pre-cleaning the substrate surface, removing xe2x80x9cnative oxidesxe2x80x9d, removing loosely-bound atoms, intermixing the substrate and film atoms and other effects, which collectively may improve film adhesion, environmental degradation rate, nucleation/seeding of desired thin film morphologies and other properties.
It would be desirable to retain as many benefits of IAD as possible while eliminating certain drawbacks. Many practitioners of thin film deposition art consider IAD too violent and damaging when applied to highly sensitive, damage-prone substrates. Damage-prone substrates include semiconductor laser diode emission facets, metal layers adjacent to electron tunnel-barrier in magnetic tunnel junctions, electron tunnel-barrier layers themselves, advanced transistor conduction channels when a gate dielectric is to be deposited on them, advanced transistor gate dielectrics themselves and advanced transistor gate metal contacts. Generally, the critical zones of these devices are on the order of 0.5 to 5 nm (5 to 50 xc3x85) in depth and at the deposition surface.
With respect to these devices, practitioners typically fear two aspects of IAD. These are momentum transfer effects and electric charge effects. Regarding momentum transfer damage to sensitive substrates, practitioners would prefer surface collision energies to be below xcx9c5 eV, to avoid disruption of the substrate lattice, intermixing and/or sputtering. Yet the dissociation energies of the reactive IAD ions, O2+, N2+, H2+, etc. lies between 5 and 10 eV, and the kinetic energy of the incident ions must be at least approximately equal to these values or there is not enough energy to dissociate the chemical bond. At these low energies, the dissociation fraction of these species is  less than 10%, and, in practice, to achieve reasonably high dissociation efficiencies, the collision energy must be 5 to 10 times higher. In this higher (25 eV to 100 eV per ion) kinetic energy range, it is justifiably expected that significant momentum-transfer damage may occur, either by lattice disruption, intermixing or sputtering.
Regarding electric charge damage to sensitive substrates, the arriving ions may build up a macroscopic positive charge on the surface of an insulating substrate, which can lead to dielectric breakdown with attendant local heating or lattice disruption. In addition, these typical IAD ions, including non-reactive Ar+ assist ions, induce a phenomenon called Auger neutralization microscopically at the surface at the location each ion collides with the surface. In Auger processes, the ion abstracts an electron from the atoms at the surface with the result that an amount of energy equal to the ionization potential of the ion (or the molecule which was ionized to form the ion) minus the binding energy of the electron abstracted from the surface atom must be dissipated. Most of this energy is dissipated to the surface atoms, potentially causing lattice disruption and/or local heating. Both types of electric charge damage are avoided if ions are simply not used.
A number of authors report the fact that reactive neutral atoms, radicals and molecular fragments may be produced in the gas phase (within a plasma) by various processes (electron impact excitation being the main one), and that these atoms, radicals and molecular fragments may find their way to the surface of the growing film and participate in film growth. Reactive neutrals in the plasma are either formed at low translational kinetic energy or become thermalized to low translational kinetic energy due to collisions with the background gas. Because of these same processes, all directionality of the reactive neutrals towards the substrate and the growing film is likewise absent or lost. Therefore the concept of using these reactive neutrals produced in the gas phase in place of IAD ions fails. H. F. Winters [xe2x80x9cElementary processes at solid surfaces immersed in low pressure plasmas,xe2x80x9d In: Topics in Current Chemistry, Vol. 94, p. 69, M. J. S. Dewar, et al (eds.), Berlin, Springer-Verlag, 1980] states in Sect. 2.2.6.2 (p. 106 ff) that, when a molecular ion collides with a solid surface, it is usually dissociated into its various constituent atoms and that some of these atoms (or radicals) are reflected away from the target. He further speculates that these reflected atoms may be incorporated into growing films. P. Martin et al [xe2x80x9cOptical properties and stress of ion-assisted aluminum nitride thin filmsxe2x80x9d, Applied Optics 31(31) p.6734 (1992)], plus others they cite, speculate that energetic back-reflected neutrals, including N atoms, from a sputtering target may contribute to some anomalous film stress results they obtained. The research group of K. W. Hipps [e.g., L. Huang, X.-D. Wang, K. W. Hipps, U. Mazur, R. Heffron and J. T. Dickinson, xe2x80x9cChemical etching of ion beam deposited AlN and AlN:H,xe2x80x9d Thin Solid Films 279 p.43 (1996)] deposited aluminum nitride and silicon nitride in numerous studies using single-ion-beam sputter deposition. Since room temperature thermal N2 gas does not react with either aluminum or silicon to form a nitride, it can be inferred that the main source of reactive nitrogen in their deposition apparatus was back-scattered N atoms from the target (N could also be implanted into the target and re-sputtered into the film).
It is believed that a flux of neutral reactive atoms (or radicals or molecular fragments) directed to the surface of a depositing thin film with kinetic energy of approximately 1 eV to approximately 30 eV would confer almost all the benefits of IAD and avoid most of the drawbacks of IAD. Providing a practical, efficient, cost-effective system and method for obtaining this is an object of the present invention. It is further believed to be beneficial that the flux of neutral reactive atoms traverse the distance from the atom source to the surface being treated with substantially no gas phase collisions, thus preserving the directionality and kinetic energy distribution of the atoms arriving at the surface. Obtaining this is a further object of the present invention. Finally, it is believed to be desirable that the intensity of the neutral atom flux be commensurate with industrial thin film deposition rates of approximately 0.1 to 1 nm/sec and that this flux be substantially uniform over the surface of substrates of 200 mm to 300 mm diameter. Obtaining this is a further object of the present invention.
A variety of sources for energetic, directional, reactive atoms (or radicals) appear in prior art, but none of these meet all of the objects of the present invention. M. Ross et al [M. M. Ross, R. J. Colton, S. L. Rose, J. R. Wyatt, J. J. DeCorpo and J. E. Campana, xe2x80x9cOn the development and use of fast atom beams for the SIMS analysis of polymers and insulators,xe2x80x9d J. Vac. Sci. Technol. A 2(2), p.748 (1984).] formed an energetic neutral beam by (partially) neutralizing an accelerated ion beam on a metal surface. The method of Ross et al, and others they cite, are typical of a number of techniques suitable for analytical instrumentation or fundamental research purposes, in that the atom flux is quite low and the spot size small.
Another method involving post-neutralization of an ion beam is disclosed by Albridge et al [U.S. Pat. No. 4,775,789 Oct. 4, 1988]. Here the ion beam is mass-selected with a Wien filter to isolate O+ ions from O2+ ions, as desired, then the ions are decelerated with an electrostatic lens, to the desired kinetic energy before neutralization occurs at a controlled grazing angle at a metal plate. This apparatus produced a 5 eV O atom flux of 2xc3x971015 atoms cmxe2x88x922 sxe2x88x921, which simulates O atom fluxes encountered by spacecraft in Earth orbit, and was intended for material erosion studies in such environments. The atom beam spot size was xcx9c1 mm, and scaling up appears difficult and expensive.
Kuwano [H. Kuwano, xe2x80x9cFast atom beam techniques for BN and other hard film formations and applications to friction-reducing coatings,xe2x80x9d Materials Science Forum Vols. 54 and 55 p. 399 (1990).] used a McIlraith (saddle field type) cold cathode ion source followed by a gaseous charge-exchange cell to produce xcx9c1 KeV inert gas atom beams via resonant charge exchange of the inert gas ions in an atmosphere of the same inert gas. Kuwano used two such fast atom sources, one to sputter a target and the other to take the place of an ion assist source to bombard the surface of the growing film. Such a method provides inefficient means of dissociating molecular ions into atoms, and, in the case of using a pre-selected atomic ion beam such as N+ or O+, resonant charge exchange neutralization would be difficult owing to the impracticality of producing sufficient density of neutral N or O atoms in the charge-exchange cell while keeping the overall pressure low enough to allow passage of the fast ions/atoms out the exit of the cell.
Schultz et al [M. Lu, A. Bousetta, R. Sukach, A. Bensaoula, K. Eipers-Smith, K. Waters and J. A. Schultz, xe2x80x9cThe growth of cubic boron nitride on Si(100) by neutralized nitrogen ion bombardment,xe2x80x9d Appl. Phys. Lett. 64(12) p. 1514 (1994).] have constructed a very efficient and functional reactive atom source by taking the accelerated output beam of a Kaufman ion source (dual-gridded electron-impact ionization type source), ionizing, e.g., N2 gas, and directing it through a series of open-ended cones, in which both neutralization and dissociative forward scattering occurs, leading to a gently convergent neutral atom beam of controllable kinetic energy. If a divergent beam is required, the cone angles can be changed and the substrate positioned farther away, beyond the cross-over waist of the convergent beam. Ionwerks, Inc. (http://www.ionwerks.com/nabs.htm) sells a version of this neutral atom beam source (NABS) commercially. The NABS requires large Kaufman sources that are expensive, the grids wear and become electrically shorted by sputter debris and the ion current output decreases sharply as a function of ion beam energy below 250 eV due to space charge limitations within the grids.
Kleiman et al [J. I. Kleiman, Z. A. Iskanderova, Y. I. Gudimenko and R. C. Tennyson, xe2x80x9cPotential applications of hyperthermal atomic oxygen for treatment of materials and structures,xe2x80x9d Surface and Interface Analysis 23 (1995).] describe another apparatus intended to simulate xcx9c5 eV atomic oxygen encountered in Earth orbit. This apparatus produces a beam of helium, atomic oxygen and molecular oxygen via a microwave-induced plasma in a helium-oxygen gas mixture followed by supersonic expansion. It can produce a higher O atom flux than the Albridge apparatus but is limited to the kinetic energy range practically achievable by supersonic expansion,  less than xcx9c4 eV, for O atoms. One object of the present invention is to reach at least 30 eV.
Another apparatus employing supersonic expansion is disclosed by Hinchliffe [U.S. Pat. No. 5,821,548 Oct. 13, 1998]. In this apparatus, a corona discharge of 10,000 to 30,000 volts DC is maintained in a gas between the expansion nozzle and a second electrode, while a downstream skimmer serves to isolate a portion of the expanding plasma and allow it to reach a substrate to be treated. In this system, the plasma (ions and electrons) reaches the substrate, the kinetic energy ranges of the atomic/molecular/ionic particles reaching the substrate is not specified, and the means of control thereof is not defined.
Another device intended to supply hyperthermal O atoms to simulate Earth orbit environment is disclosed by Outlaw et al [U.S. Pat. No. 5,834,768 Nov. 10, 1998 and self-cited prior art U.S. Pat. No. 4,828,817 May 9, 1989]. In this device oxygen molecules are supplied to one side of a hot metal membrane and dissociated O atoms therefrom diffuse through the metal and appear as O atoms adsorbed on the other side of the membrane. From there, a portion of the O atoms are energetically desorbed by electron-stimulated desorption (ESD) or photon-stimulated desorption (PSD) and are used to treat a substrate. However, both ESD and PSD, and the kinetic energy with which the O atom is ejected, are controlled by atomic and solid-state electronic structure and energetic effects. As such, the kinetic energy of the ejected O atoms is not continuously variable and is specific to the chemical identity of each O-metal couple. Moreover, the kinetic energies of ejection are low, i.e., a few eV at most. Also, in a high-rate, mass-production deposition apparatus, the substrate side of the metal membrane might become coated with stray coating intended for the substrate, thus potentially xe2x80x9cpoisoningxe2x80x9d the inner surface of the membrane for either surface accumulation of O atoms or their desorption, rendering the device impractical in this application.
The present invention is directed to a system and method for forming a chemically reacted layer proximate an exposed surface of a substrate. A gas supply provides a chemically reactive molecular gas to an ion source that generates a divergent ion current directed at a target. The ion current contains at least one species of chemically reactive molecular ion, and the target is disposed in a chamber having a partial vacuum in the range of 10xe2x88x922 to 10xe2x88x925 Torr. A voltage source applies a bias to the target such that chemically reactive molecular ions from the ion source are accelerated toward the target with sufficient kinetic energy to dissociate at least some of the chemically reactive molecular ions by collision with the surface of the target to form a population of neutral chemically reactive molecular fragments, atoms or radicals at least some of which scatter away from the surface of the target and into the chamber. At least a portion of the population of neutral chemically reactive molecular fragments, atoms or radicals is intercepted at an exposed surface of a substrate disposed in the chamber. The neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate have a kinetic energy that is a function of a value of the bias. The chemically reacted layer corresponds to a product of at least one chemical reaction of the neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate with other atoms proximate the exposed surface of the substrate. In one embodiment, the chemically reacted layer is formed without bombarding the exposed surface of the substrate with ions.
In accordance with further aspects, a controller may vary the value of the bias applied to the at least one target, and thereby vary the kinetic energy of the neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate. In one embodiment, the controller continuously varies the value of the bias applied to the at least one target, and thereby continuously varies the kinetic energy of the neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate over a range from 1-30 eV or higher. The controller can also optionally vary an angle of the substrate such that an arrival angle of the neutral chemically reactive molecular fragments, atoms or radicals intercepted at the exposed surface of the substrate varies from a substantially normal angle of incidence to a substantially grazing angle of incidence. An electron source may also be disposed in the chamber to provide electrons that neutralize accumulated ion-charge build-up on the target.
In accordance with a further aspect, the chemically reactive molecular ions accelerated toward the target cause atoms from the surface of the target to be sputtered away from the target, and a portion of the atoms sputtered from the surface of the target collect upon the exposed surface of the substrate with the neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate. A thin film formed on the exposed surface of the substrate includes a chemical compound or alloy resulting from reaction of the atoms sputtered from the surface of the target with the neutral chemically reactive molecular fragments, atoms or radicals intercepted by the substrate.
In a further embodiment, first and second targets are disposed in the chamber. In this embodiment, chemically reactive molecular ions accelerated toward the first target cause atoms from the surface of the first target to be sputtered away from the first target, and chemically reactive molecular ions accelerated toward the second target cause atoms from the surface of the second target to be sputtered away from the second target. Portions of the atoms sputtered from the surfaces of the first and second targets collect upon the exposed surface of the substrate with neutral chemically reactive molecular fragments, atoms or radicals scattered from the first and second targets and intercepted by the substrate. A thin film formed on the exposed surface of the substrate includes a chemical compound or alloy resulting from reaction of the atoms sputtered from the surfaces of the first and second targets with the neutral chemically reactive molecular fragments, atoms or radicals scattered from the first and second targets and intercepted by the substrate. A controller may vary or control properties of the thin film by controlling arrival rates at the exposed surface of the substrate of the atoms sputtered from the surfaces of the first and second targets and the neutral chemically reactive molecular fragments, atoms or radicals scattered from the first and second targets.
In the two target embodiment, a first ion source may be used to generate a first divergent ion current directed predominately at the first target and a second ion source may be used to generate a second divergent ion current directed predominately at the second target. The first divergent ion current optionally includes a first species of ions that is different from a second species of ions in the second divergent ion current. For example, the first species may comprise inert gas ions while the second species comprises reactive molecular gas ions. Alternatively, the first and second species may respectively comprise first and second species of reactive molecular gas ions.
In the two target embodiment, the controller may independently control a quantity of the first species of ions directed at the first target and a quantity of the second species of ions directed at the second target. Similarly, the controller may independently control a first magnitude of flux of neutral chemically reactive molecular fragments, atoms or radicals scattered from the first target and a second magnitude of flux of neutral chemically reactive molecular fragments, atoms or radicals scattered from the second target. In some cases, a chemical composition of the thin film depends at least in part on a ratio of the first magnitude to the second magnitude. In these cases, the controller may vary the ratio over time such that the chemical composition of the thin film formed on the substrate varies within a thickness of the thin film. For example, the chemical composition of the thin film formed on the substrate can be made to vary linearly, in a sinusoidal or parabolic fashion, or as a step-function, within the thickness of the thin film.
In the two target embodiment, the first target may be biased with a different voltage amplitude than the second target, and the controller may independently vary the biases applied to the first and second targets. Also, the base material of the first target may be the same as or different from a base material of the second target, and an amount of material sputtered from the first target may be the same as or different from the amount of material sputtered from the second target.
In accordance with a further aspect applicable to single or multi-target embodiments, the bias applied to the target corresponds to a pulsed DC voltage signal (e.g., a bi-polar DC voltage pulse signal or an a-symmetric bi-polar DC voltage pulse signal), and the controller varies a flux of the neutral chemically reactive molecular fragments, atoms or radicals scattered from the surface of the target by varying a width, amplitude or frequency of negative pulses in the DC voltage pulse signal.